High accuracy weathering test machine

ABSTRACT

A weathering testing machine or the like is provided with improved stabilization of conditions such as temperature and humidity, coupled with more accurate readout from sensors on a moving member in the machine, and other sensors and microprocessors.

BACKGROUND OF THE INVENTION

Systems for testing materials, for example the weathering andlightfastness of products such as fabric samples, painted panels, andplastics, are presently available, being sold for example by the AtlasElectric Devices Company of Chicago, Ill. Also, such devices aredisclosed in Suga U.S. Pat. No. 4,627,287, and Huber et al. U.S. Pat.Nos. 4,843,893 and 5,226,318, for example. Weathering testing devicestest the weathering and lightfastness properties of materials andproducts under closely controlled conditions.

In the natural environment, heat, light, and moisture combinesynergistically to cause optical, mechanical, and chemical changes inproducts which are exposed to outdoor weathering conditions. Typically,the testing apparatus of this invention and the prior art can be used toobtain such weathering data on an accelerated time basis, to permitproduct manufacturers to gain information as to how their products willstand up to weathering over the months or years.

Typically, a weathering testing apparatus may use air which circulatesthrough the system to control the temperature of samples being tested,so that they are not underheated or overheated by a heater or radiationsource which may be present, typically a high intensity plasma lamp suchas a xenon lamp. It is desirable for the samples being tested to beexposed to precisely predetermined conditions, to permit more accuratecomparison between various testing runs and the like, and so that theweathering conditions provided by a weathering tester can be accuratelypredetermined and thus recreated when desired for comparison of varioussamples over the years, and the like.

By this invention, improvements are provided which increase the accuracyof the testing machine of this invention, in that the machine can beused to provide accurately predetermined conditions which aresubstantially predictable and invariant throughout a run and from run torun, and also in that temperature and other measurements may be moreaccurately made, even from sensors which are carried on a moving testrack carried in the enclosure of the testing machine.

By this invention more accurate results, with better reproducability,can be obtained concerning the lifetimes of materials and other testingissues.

DESCRIPTION OF THE INVENTION

By this invention, in one aspect thereof, a method is provided ofcontrolling temperature in a testing chamber having a heater and avariably openable and closeable air inlet vent. The vent is controllableby a first controller and a first temperature sensor in the chamber. Thecontroller serves to open and close the vent responsive to thetemperature sensed by the first sensor. By the method of this invention,one selects, preferably automatically, a first vent position that ismore open than a mean or average vent position that provides a desiredtemperature in the chamber under a first set of operating conditions, asdetermined by the first controller and first temperature sensor. Onethen fixes the vent in that first position. Thereafter, one controls thetemperature entirely by the heater.

The advantage of this invention can be seen from the followingbackground: In Huber et al. U.S. Pat. No. 4,843,893, a weatheringtesting system is shown which has a temperature sensor thatautomatically controls the position of a damper or vent in an aircirculation flow path which circulates air through the chamber of theweathering testing system. Thus, as the temperature rises, it is sensedby the sensor. The sensor will signal for the damper to be opened to adegree that causes more cooling air to enter the system, thus reducingthe temperature and providing a feedback loop control.

In other prior art systems, both the damper and the heater arecontrolled by a single loop of a controller, so that the damper opensand the heater reduces its production of heat as the temperature rises,while the damper closes and the heater increases its heat production asthe temperature drops.

Testing systems having chambers with these types of control systems tendto have temperatures that oscillate, as the damper and the heater workin conjunction with the sensor in a continuing attempt to stabilize thetemperature of the chamber at a desired level. Of course, the turbulenceof new air brought in from the damper causes nonstandard temperatureregions to briefly form. Likewise, humidity can be significantlyaffected by the operation of the damper, so that undesired oscillationsin both humidity and temperature take place, typically about a meanvalue. This causes corresponding oscillations of the degree of damperopening and heater activity about mean values.

By the method of this application, the approximate mean (or average)position of the oscillating damper is determined for obtaining a desiredtemperature at the specific operating conditions which are being used inthe weathering testing machine. Such operating conditions include thedesired temperature, the desired humidity, the blower speed used, theintensity of the radiation source used, and the like.

One then fixes the position of the damper in a first vent position thatis slightly more open than the mean or average of the range ofoscillating damper positions in the vent which are conventionallyprovided by the first controller and first temperature sensor in thechamber. This mean or average position and the slightly more openposition can be automatically computed.

This immediately stabilizes the conditions in the operating testingchamber, but typically going toward an equilibrium temperature that isslightly cooler than desired.

The first vent position is typically only slightly more open than themean or average vent position, with the first vent position desirablyproviding no more than about 10 percent more cooling air to the chamberthan is required to maintain the desired temperature under a first setof operating conditions of the testing chamber. Thus, to maintain thedesired temperature, the heater will tend to produce a little more heatthan in the prior art mode of operation under similar conditions, butsignificant improvements in the stability of the conditions will beachieved, for testing results of improved accuracy.

The heater can be controlled by the first controller, responsive to thesame signals from the first temperature sensor as is the damper, ifdesired, for a simplification of parts and circuitry.

Also, it is preferred for a blower motor to be present to blow airthrough the chamber, and for second controller and a second temperaturesensor to be present, to control the blower motor speed dependent uponsignals from the second temperature sensor. Typically, the firsttemperature sensor is positioned to sense primarily air temperature,being remote from a central, radiant energy source in the chamber. Thesecond temperature sensor may be a black panel sensor, positioned toprimarily sense temperature directly imparted by the radiant energysource in the chamber. Thus, as previously described in U.S. Pat. No.4,843,893, simultaneous and somewhat separate temperature control ofdark testing samples and light testing samples can be achieved, but bythis invention under conditions which are very stable and which can beaccurately reproduced.

In the event that the first temperature sensor senses a temperature thatmoves out of specified limits, indicating a loss of temperature control,the software of the system within the first controller can cause thedamper to be released, to find a new oscillating set of positions ofvariable damper opening, and accordingly a new, mean damper position,provided by the first controller and temperature sensor in conventionalmanner. Then, after determining a new, mean position for the damper, thecontrol can automatically select a slightly more open position asbefore, which in this circumstance is likely to be different from theearlier slightly more open position, and the damper can be set instationary manner in the new position. Once again, conditions will besubstantially stabilized, with the temperature control being governedexclusively by variations of temperature output in the heater.

Thus, a continuing, dynamic control system may be provided in which theposition of the damper vent is locked in a stationary position asdescribed above, except when the sensed temperature moves out of aspecified range.

Then the system unlocks, and goes back to its original mode oftemperature control as described above, following which the damper ventmay be relocked in a new position for high stability temperature controlwhenever the damper vent is locked in a particular position.

Further in accordance with this invention, the testing chamber may beequipped with a source of humidity, typically a water spray nozzle as isconventional. By this invention, an improved method of providing andcontrolling the humidity levels to the testing chamber is provided, inwhich the testing chamber has a blower for circulating air in thechamber. By this invention, one provides a water spray nozzle, asstated, extending into the circulating air of the chamber. One thendefines a unit of time, with the process of this invention beingrepeated in sequential units of time as defined. Typically, such a unitof time is no more than about thirty seconds and preferably no more thanabout 10 seconds.

One periodically determines the humidity of the air in the chamber. Onethen compares the difference of the humidity of the air in the chamberwith a desired or set humidity objective. The water spray is applied tothe chamber through the nozzle for a portion of the unit of time down tozero percent, which portion is dependent on the compared humiditydifference as described above, which may be electronically compared andcalculated.

The water spray is not provided or applied to the chamber for theremainder of the unit of time. For example, 5 to 30 percent of the unitof time may comprise the "on" mode of the spray nozzle, in which sprayis being inserted into the chamber, and the balance of the unit of timemay comprise the "off" mode, in which water spray is not being added tothe circulating air of the chamber.

The process is repeated in subsequent units of time, preferably in eachadjacent, sequential unit of time, to provide a consistent, pulsedapplication of water spray to the circulating air throughout theoperation of the testing chamber. However, if desired, longer gaps inthe spray application over time may be provided if desired.

By this invention, a significant improvement in the uniformity of thehumidity imparted to the chamber may be achieved. The specific humidityimparted may depend if desired on a single variable: the percentage ofunit of time in which the spray is activated. Such high uniformityprovides excellent reproducability of humidity conditions from testingrun to testing run, plus the precise humidity that is desired, withoutsignificant variation due to unstable and non-standard conditions.

Particularly, the uniformity of humidity provided by this inventioncomprises a substantial improvement over methods in which a humiditysensor activates a water spray or atomizing nozzle for continuousoperation when the sensor senses an undesirably low humidity, and thenshuts off the water spray when the sensor detects proper humidity. Insuch procedures, there is often a humidity overshoot, in which thehumidity rises beyond the desired level in non-uniform manner, contraryto the humidification method disclosed herein.

In similar manner, by this invention a heater in a testing chamberhaving a blower for circulating air in the chamber may provide heat tothe system in accordance with the following method: one defines a unitof time similar in concept and use to the unit of time previouslydiscussed with respect to the humidification process. One periodicallydetermines the temperature of the air in the chamber, then comparing thedifference of the temperature of the air in the chamber with a desiredor set temperature. One activates the heater (by turning on the electricpower if the heater is electric) for a portion of the unit of time,which portion is dependent upon the compared temperature differencedescribed above. The particular portion may by computed electronicallyin a manner believed to be readily understandable by those skilled inthe art. Then, one does not activate the heater (e.g. the power is off)for the remainder of the unit of time.

Here also, this process is repeated in subsequent units of time toprovide a pulsed application of the heater, typically over a continuingseries of adjacent units of time, which units are typically no more thanabout 30 seconds in length and preferably no more than about 10 seconds.Thus, for example, a heater may be activated for one or two seconds outof every 10 second period of time that elapses throughout the testingprocess that is taking place in the testing chamber. Such an applicationof heat provides high uniformity of temperature conditions throughoutthe testing chamber.

As previously described, the heater is responsive in its operation tothe first sensor and first controller. When the air temperature issensed to be falling below the desired level, the controller can causethe heat energy to be provided to the heater for a larger portion ofeach unit of time. If the temperature rises above the desired limits,the heat energy can be provided to the heater for a smaller portion ofeach unit of time. The same principle applied in thepreviously-described humidity control process.

Thus, further improvements in the uniformity of test chamber conditionscan be achieved.

As a further embodiment of this invention, a weathering testing systemis provided which comprises a chamber and a rack for carrying samples tobe tested. The rack, in turn, defines a central space. A lamp is locatedcentrally in the rack for irradiating samples carried on the rack. Ablower is provided for directing a stream of air through the rack.

The chamber has top and bottom walls. The rack is carried by a supportmember, typically a rotatable shaft, which extends through the topchamber wall. The lamp is carried by a second support member whichextends through the bottom chamber wall, with the lamp being spaced fromthe top chamber wall and thus isolated therefrom. Typically, the rack isspaced from the bottom chamber wall.

The typical electronic sensors (such as temperature, humidity, lightsensors) which are carried by the rack may communicate along or throughthe first support member, which extends through the top chamber wall,being thus isolated from the heavy electric currents and cooling waterwhich are typically provided to the lamp. The lamp electric circuitryand any cooling water circuitry present extends downwardly along orthrough the second support member, passing through the bottom chamberwall.

This provides the sensitive circuitry and sensors which may be carriedby the rack, with isolation from the heavy electric currents of thelamp, reducing the electrical noise encountered in the sensitive sensorcircuits and the like of the rack, and also isolating the electroniccomponents associated with the rack from any water that escapes fromWater lines.

Thus, the data gathering capability of the testing machine of thisinvention can be improved over prior art systems by the reduction ofelectrical noise as well as a reduction of damage, caused by inadvertentwater migration, to the electronics of the system.

Further in accordance with this invention, an improved method ofdetermining data on a moving test member is provided. Specifically, therotating rack described above may comprise the moving test member, withthe data being picked up by the first and second sensors, or otherdevices, as may be desired.

In accordance with the method, one operates an electric circuit which isat least partially carried on the test member. The circuit comprises asensor device having a resistance that varies as a function of the datato be determined, for example a variable resistance temperature sensor,to provide a first signal in the circuit which carries the data. Onethen converts the first signal into a variable current signal in whichthe current of the signal is a function of the first signal, so that thevariable current signal carries the data to be determined inthe-variability of the current.

One then passes the variable current signal through a typicallyconventional collector or collectors from the moving test member to astationary control system, which is spaced from the test member. Thecurrent variations of the variable current signal are then convertedinto the data in a useable form, for example a numerical data value ofthe temperature sensed, in degrees Celsius.

By this technique, it becomes possible to provide the data to the userin an absolute, quantitative, error-free form rather than merely arelative data readout.

Prior art systems for determining data from a moving rack in a testingchamber provide a voltage dependent signal as a function of the variableresistance of the sensor device described above. This voltage dependentsignal is passed through a collector to the stationary control system,with the data being a function of the voltage sensed.

However, the voltage can be varied by the resistance of the collector,which can change from day-to-day, so that the data provided may haveerrors, requiring frequent calibration of the system against anobjective standard. One reason for this is that collectors, which areknown devices for passing electric current from a moving system to astationary system using brushes against a moving surface or an electrodein contact with mercury, will vary in their resistance with time. Thisresults in significant voltage variations of the signal as a function-ofthe collector itself, which renders the data inaccurate, and onlyuseable on a comparative basis.

By this invention, for the first time in the field of testing chambers,error-free temperature and other data may be acquired from devicescarried on a rotating rack or the like. This is accomplished asdescribed above by providing a signal which leaves the moving rackthrough a collector and is directed to the stationary control system,which signal has a controlled current (in amperes) which dependsentirely upon the resistance value which is found in the variableresistance device. Through the conventional electronics used, thevoltage of the signal may vary or be constant as necessary to achievethis predetermined current for the signal. Thus, an objective value thatis a pure function of the variable resistance sensor is provided to thecontrol system, where it may be conventionally converted into anumerical readout of temperature or the like, being an absolute valuewithout need for correction of errors introduced by the electronics.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a weathering testing system inaccordance with this invention;

FIG. 2 is a perspective view showing the upper portion of the uppercompartment of the weathering tester of FIG. 1;

FIG. 3 is a perspective view showing a lower portion of the uppercompartment of FIG. 1; and

FIG. 4 is a diagrammatic view of a sensing system in accordance withthis invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, a weathering testing device 10 is shown,which comprises a housing 12 defining an upper chamber 14 in which arack 16 resides, comprising a roughly spherical array of stainless steelstruts, to which test samples 18 may be attached in a mannersubstantially equidistant from a central light source 22, which may be axenon lamp. This arrangement is similar to that disclosed in U.S. Pat.No. 4,843,893.

At the bottom of upper chamber 14, a circular arrangement of apertures26 are provided, plus a conical baffle 24 to assist in directing airpassing through apertures 26 along the test samples 18 carried on therack.

A conventional resistance type heater element 30 is positioned under theapertures 26 and the partition that carries them. Also, a fitting 32(FIG. 3) for xenon lamp 22 is provided, being adjustable and designed inaccordance with U.S. Pat. No. 5,226,318. Fitting 32 includes bothelectrical and water flow conduits for servicing the xenon lamp 22,while xenon lamp 22 is spaced from the top of upper chamber 14. Lampfitting 32 is also named the "second support member" above.

Rack 16 is carried by a first support member or shaft 34 which extendsthrough the top wall 36 of the upper chamber 14. Thus, the connectionsof various electronic devices carried on rack 16 may pass with shaft 34through top wall 36 to a microprocessor 38 (FIG. 2) that is carried onthe weathering testing system above top wall 36, in a manner that issafely spaced from both the flowing water and the high electric currentsand voltages used with respect to xenon lamp 22.

Also, a motor M is positioned above top wall 36 which rotates shaft 34and rack 16, when that is desired during the weathering testing process.

Test rack 16 may carry a black panel temperature sensor 40, which is asensor particularly adapted to sense the temperature directly impartedby the radiation from the xenon lamp. This sensor models the temperatureconditions encountered by darker samples carried on the rack. Dry bulbsensor 41 is provided in a position more remote from lamp 22, to monitorair temperature. Also, a direct percentage RH humidity sensor 43 isprovided. Each of these provide signal data to microprocessor 38.

Top wall 36 also defines wall apertures 44 which represent the inlet ofa circulatory plenum 46 that circulates air, driven by blower 28, fromthe top to the bottom of chamber 14 and through apertures 26, aspropelled by blower 28.

Within plenum 46 is a variably openable cooling air supply vent 48having a movable damper 50, and comprising air inlet 48b and air outlet48a. The position of the damper 50 can be controlled by a control member51 which is, in turn, controlled by the microprocessor 38 in aconventional manner.

Rack water spray or atomizer unit 52 is also provided in upper chamber14, along with a light sensor 54, which directly monitors the radiationemission of the xenon lamp. Also, a specimen water spray or atomizerunit 53 is provided for added specific spraying of the specimens whenthat is desired.

In accordance with one aspect of this invention, lamp 22 is mounted in abottom fitting 32 which may be similar in structure and function to thecorresponding fitting disclosed in U.S. Pat. No. 5,226,318, except thatfitting 32 is mounted on the bottom wall of chamber 14 rather than thetop wall so that lamp 22 extends upwardly therefrom and is spaced fromthe top wall 14.

Also, as shown, rack 16 is suspended from the top wall 14, being carriedby shaft 34, and being spaced from the bottom wall of the chamber.

Accordingly, as previously described, the electronics of the system,exemplified by microprocessor 38, can be well spaced from the highvoltage and amperage inlet and water inlet which feeds from the bottomof the chamber upwardly in otherwise conventional manner, to space thehigh electric currents and water from the sensitive electronics of thesystem.

Further in accordance with this invention, one controls the temperaturein testing chamber 14, making use of heater 30 and a variably openableand closeable cooling air inlet vent 48 which has a damper 50 which isvariably openable and closeable by a first controller 51. The firstcontroller comprises a controller motor which is operated by signalsfrom microprocessor 38. Microprocessor 38, in turn, processes signalsfrom typically temperature sensor 41 to cause the damper 50 to assume aposition intended to stabilize the circulating air temperature at adesired value.

However, as previously described, what typically happens in actuality isthat the temperature readings oscillate, causing the damper to oscillateabout a mean vent position. The effect of this is an undesirableinstability of air temperature.

In accordance with this invention, microprocessor 38 determines a meanor average vent position in an empirical manner over a specified periodof time, perhaps two to three minutes, this being accomplished by anymanner which is obvious or desirable to those skilled in the art ofcomputer programming. Then, a first damper position is selected by themicroprocessor, which preferably places the vent about 10 percent moreopen than the computed mean position, and the damper is temporarilyfixed in that new position without further motion. Thus, as the aircirculates through plenum 46, impelled by motor 28, the temperatureoscillations caused by the feedback between the signals sent bytemperature sensor 41 to processor 38, and the oscillations of damper50, are stabilized, although at a damper position which can be expectedto cause more fresh air to enter the system than that amount required tomaintain the desired temperature under the particular conditions ofoperation such as blower speed, lamp energy production, and the like.

Thereafter, heater 30, which also may be controlled by microprocessor38, serves as the only temperature control in an otherwise-stablesystem, so that precise, uniform, reproducible air temperature controlcan be provided in a system of stable variables except for theproduction of heat, at a stable damper setting that closely approximatesthe damper setting appropriate to the desired temperature. This can beaccomplished through an automated system which spontaneously determinesthe desired damper position and which spontaneously controls the heater30 to provide a stable, predetermined temperature.

If the temperature is sensed by unit 41 to go out of a predeterminedrange, the software of microprocessor 38 once again releases the fixedposition of damper 50, to go back into the feedback mode where thedamper is responsive to moment-by-moment temperature signals from sensor41. An oscillation of the damper 50 is once again likely to occur for aperiod of time until a new mean position for the damper is determined.At this time the process of this invention takes place again by fixingof the damper 50 at a predetermined position wider open than the meanposition of the oscillating damper, to achieve the desired resultsdiscussed above.

Black panel sensor 40 also communicates by a wire which passes throughor beside shaft 34, for electrical contact with microprocessor 38.The-speed of blower 28 can, if desired, be controlled in a mannerresponsive to the temperature signals received from black panel sensor40.

As the temperature in tester 10 originally warms up a minimumtemperature set point may be used, at which point damper 50 opensslightly and is set to stay at least slightly open (about 5 percent)while the temperature is above the set point, to facilitate thetemperature control through the blower 28 and back panel sensor 40.

Further in accordance with this invention, heating coil 30 can also becontrolled in an automated manner from temperature signals received bysensor 41. This is accomplished by defining time into discrete, adjacentunits of typically 5 seconds each. Sensor 41 sends temperature signalsto microprocessor 38 which compares the signals with a desiredtemperature, and determines the difference of the temperature of the airin the chamber with that desired temperature. If the difference is zero,or sensor 41 senses a temperature higher than the desired temperature,then the microprocessor 38 does not activate heater coil 30 during theunit of time. However, if the microprocessor 38 determines that thetemperature sensed by sensor 41 is below the desired temperature, then,the microprocessor will cause heater coil 30 to be electrically actuatedfor a particular percentage of each 5 second unit of time. Thispercentage may be directly dependent on the size of the temperaturedifference determined by the microprocessor, so that if the temperaturesensed is 10 degrees or more under the desired temperature, then theheater coil 30 is energized for 100 percent of each 5 second incrementor unit of time. However, as the difference between the sensed and thedesired temperature gets smaller, the fraction of each unit of time inwhich the heater coil is energized for production of heat goes downuntil, when the sensed temperature and the desired temperature are thesame, the heater coil 30 is energized for a predetermined fraction ofeach unit of time that tends to maintain the desired temperature.

By this means, accurate and reproducible temperature control with lowdeviations from the desired temperature can be achieved, avoiding thetemperature overshoot of conventional control schemes.

In similar manner, the humidity provided to the circulating air in thechamber through spray or atomizer units 52, 53 can be controlled.Humidity sensor 43 communicates with microprocessor 38. Themicroprocessor 38 defines sequential units of time, which typically maybe 5 second increments as in the previous control method fortemperature. The difference between the sensed humidity and apredetermined humidity is determined. If the difference is zero, one orboth of the spray nozzles 52, 53 are "on" during only that portion ofeach 5 second repeating unit of time needed to maintain the desiredcondition, for example 20 percent of the time. Similarly, if the sensedhumidity is higher than the predetermined humidity set in themicroprocessor, the respective sprays may be off for each entire unit oftime.

However, if the sensed humidity drops below the predetermined humidity,then the microprocessor 38 will signal one or both of sprays 52, 53 toturn on for an increasing portion of each of the respective units oftime, with that portion increasing to 100 percent of each unit of timeas the difference between the predetermined humidity and the sensedhumidity is larger. Thus, a continuing but intermittent spray isprovided for substantial stabilization of the humidity at apredetermined level, with greatly reduced fluctuation in the humidity asprovided by the prior art techniques for applying water spray to thecirculating air. This can be accomplished in a fully automated manner.

Further in accordance with this invention, signals from sensors carriedon the rotating rack 16, for example black panel sensor 40, must passacross an electrical collector 60 carried about shaft 34 to permitelectric signals to pass from the moving sensor 40 to the stationarymicroprocessor 38 or other electronic monitor and control system carriedon the frame of the apparatus.

The conventional technique of accomplishing this uses a black panelsensor that may comprise a resistance temperature device such as a knownRTD PT-100 unit, in which the electrical resistance thereof varies inaccordance with the temperature sensed. The moving resistancetemperature device (RTD) is joined in an electrical circuit with astationary electric monitor and control system. The circuit passesthrough a pair of collectors, which may be standard devices for passingan electric signal from a moving member to a stationary member. Forexample, a collector using brushes or liquid mercury is well-known. Theelectronic monitor and control system monitors the voltage in theelectrical circuit, which of course is dependent on the variableresistance of the resistance temperature device, to determine thetemperature sensed by the resistance temperature device.

However, a significant problem in this prior art system lies in the factthat the resistance of the collector can also vary with use thereof, aswell as with other factors. Thus, this system only provides a relativetemperature value so that the electronic system must be frequentlycalibrated.

By this invention, an electronic system and method are provided fordetermining data in a moving test member, in which the data provided toa stationary control system outside of the moving test member isquantitative, without the need for frequent calibration.

By this invention, referring to FIG. 4, one operates an electric circuit59 having a portion 61 that is carried by the moving rack 16. Thiscircuit portion comprises a resistance temperature device 62 which maybe identical to the device used in the prior art, as part of the blackpanel sensor 40. Device 62 is electrically connected in a circuit to anelectronic block 66, as a new element of the invention, which measuresthe resistance in the resistance temperature device 62 and produces asignal current in response thereto which is directly and exclusivelyresponsive to the variable resistance of RTD 62, under the specificconditions of use. Such a device is known, and may be accomplished bymany different electronic designs. Specifically, a Yakagawa signaltransmitter (RTD to Current) JUXTA FR5A may be used as electronic block66.

Electronic block 66 is connected in a circuit 68 with electronic monitorand control system 64 through collectors 60 which, as previouslydescribed, are conventional, and may be either brush collectors, liquidmercury collectors, or the like, to permit a moving electrical terminalto communicate with a stationary electrical terminal.

The signal which passes through circuit 68 between electronic block 66and control system 64 has a predetermined amperage or current, asdetermined by electronic block 66 in response to the resistance ofdevice 62, while the Voltage of the system may vary as necessary todetermine that the precise current is transmitted. Thus, any unplannedresistance variations which may be found in collectors 60 do notinterfere with the transmission of the signal from electronic block 66to control system 64. Control system 64 monitors the value of thecurrent in circuit 68, ignoring voltage variations, to provide anabsolute, essentially error free readout that is a function of thevariable resistance of RTD 62. Accordingly, a precise, quantitativetemperature is sensed in accordance with this invention.

Control system 64 may comprise a Fanuc 90-30 PLC controller.

Accordingly, this invention may exhibit unparalleled improvements inaccuracy of both data readout and precise creation of desired conditionsof temperature and humidity in weathering testing systems, withautomated control, but without undesirable fluctuations in theconditions due to feedback problems. Thus, weathering conditions can beprecisely duplicated from experimental run to experimental run,facilitating the comparison of results taken in different runs and atdifferent times.

The above has been offered for illustrative purposes only, and is notintended to limit the scope of the invention of this application, whichis as defined in the claims below.

That which is claimed is:
 1. The method of controlling temperature in atesting chamber having a heater and a variably openable and closeablecooling air inlet vent, said vent being controllable by a firstcontroller and first temperature sensor in said chamber to open andclose said vent responsive to the temperature sensed by the firstsensor, said method comprising: selecting a first vent position that ismore open than substantially the mean or average vent position whichprovides a desired temperature in said chamber under a first set ofoperating conditions, as determined by the first controller and firsttemperature sensor; fixing said vent in said first vent position; andthereafter controlling said temperature by said heater.
 2. The method ofclaim 1 in which said first vent position provides no more than tenpercent more cooling air to the chamber than is required to maintain thedesired temperature under said first set of operating conditions.
 3. Themethod of claim 1 in which said heater is controlled by said controller,responsive to signals from said first temperature sensor.
 4. The methodof claim 1 in which a blower and motor are present to blow air throughsaid chamber, and a second controller and a second temperature sensor ispresent to control the blower motor speed dependent on signals from saidsecond temperature sensor.
 5. The method of claim 4 in which said firsttemperature sensor is positioned to sense primarily air temperature,while the second sensor is a black panel sensor positioned to primarilysense temperature directly as imparted by a radiant energy sourcepositioned in said chamber.
 6. The method of claim 1, including the stepof releasing said vent from said fixed first position to permit the ventposition to be controlled by said first controller and first sensor whenthe temperature sensed by said first sensor is out of a predeterminedrange.
 7. The method of claim 6 in which, when said temperature sensedby the first sensor returns to the predetermined range, said vent isagain fixed into a new first vent position that is different from thethen-current mean vent position.